1. Field of the Invention
The invention relates generally to seismic surveys. More particularly, this invention relates to transducer sources for seismic surveys
2. Background Art
Seismic surveys are commonly used to profile rock formation properties. Conventional seismic surveys use surface energy sources and surface receivers to detect reflections from subsurface acoustic impedance contrasts, i.e., subsurface rock strata. However, results from the conventional seismic surveys are not always satisfactory because the sound waves have to travel long distances back and forth. Cross-well seismic surveys, in which the seismic source and the receivers are deployed separately in nearby wells, overcome some of the disadvantages of the conventional seismic surveys. However, a cross-well approach requires the drilling of additional wells, with added costs.
Vertical Seismic Profiling (VSP) provides more accurate information than conventional seismic surveys. In VSP, seismic sources are located at the surface and the sensors (e.g., geophones, hydrophones, accelerometers) are located in the borehole. VSP, however, suffers from several drawbacks, including costs associated with multiple surface energy sources and multiple ghost images due to energy trapped in the surface layer. More recently, reverse (or inverse) VSP (RVSP) was developed to overcome the logistic limitations of the multiple surface energy sources needed in some VSP surveys. In RVSP, a single seismic source is deployed in the borehole and the receivers are arranged on the surface. In “long-spacing sonic” profiling, the receivers and the source are placed in the same borehole, with a substantial distance separating the source and the receivers. Both the RVSP and the long-spacing sonic technique use seismic sources that are placed in boreholes, i.e., downhole seismic sources.
Various downhole seismic sources have been developed over the years. Some downhole sources use drill bits or drill strings as the sources. For example, U.S. Pat. No. 2,062,151 issued to Weatherby discloses a source using a drill bit as an impulse generator of seismic waves, while U.S. Pat. Nos. 4,363,112 and 4,365,322 issued to Widrow disclose sources using the natural random vibrations of drill strings to launch seismic waves.
Various other types of downhole seismic sources, not using a drill bit or drill string, are also available. For example, U.S. Pat. No. 3,909,776 issued to Broding et al. discloses a source using a fluid driven oscillator, which changes the emitted frequencies as a function of time. Similarly, U.S. Pat. No. 3,881,168 issued to Farr et al. discloses a source using a mono-frequency fluid oscillator. U.S. Pat. No. 4,207,619 issued to Klaveness and U.S. Pat. No. 4,033,429 issued to Farr disclose sources using pulse generators located in the drill string. U.S. Pat. No. 5,137,109 issued to Dorel discloses a downhole seismic source in which a body containing a resonant system is clamped to the borehole wall. In the system of Dorel, the seismic signal is applied to the borehole by clamping the source to the borehole wall.
Other downhole seismic sources include impulsive sources (e.g., Primacord™, which is a detonation cord used in blasting and is available from Ensign-Bickford Co., Spanish Fork, Utah, and air guns), swept frequency signal generators (see e.g., U.S. Pat. No. 4,671,379 issued to Kennedy et al.), and piezoelectric actuators (see e.g., U.S. Pat. No. 5,477,101 issued to Ounadjela).
While these prior art seismic sources and various survey methods (e.g., cross-well, VSP, or RVSP surveys) can provide valuable information about the formations, there exists a need for seismic sources that can be used with seismic receivers in the same wellbore. The ability to have the source and receivers in the same borehole will make it possible to perform lateral profiling and to probe the reflecting interface from above and below the interface.
One problem associated with using the source and the receivers in the same borehole relates to the transmission of the seismic signals directly from the source to the receivers via the fluid column in the borehole. The fluid column in the borehole may function as a wave guide to transmit the signals with high efficiency. These seismic signals propagating in the borehole can interfere with the detection of the desired signals. To alleviate this problem, several prior art methods have been proposed.
U.S. Pat. No. 4,858,718 issued to Chelminski discloses a method for attenuating tube waves for use with an impulsive downhole seismic source. The device uses gas-filled resilient bladders positioned above and below the seismic source to attenuate. The bladders are protected in a perforated protective housing that has a diameter slightly smaller than the diameter of the borehole. Thus, these bladders do not completely separate the fluid column into isolated sections.
U.S. Pat. No. 5,171,943 issued to Balogh et al. discloses a tube wave damper probe for the suppression of borehole tube waves in seismic applications. See also, W. T. Balogh, “The Borehole Tubewave Damper Probe,” Expanded Abstracts, SEG, 159-162, 1992. The damper comprises a gas-filled bladder disposed in the housing. The gas-filled bladder functions as a modified Helmholtz resonator to reduce the propagation of waves of certain frequencies.
U.S. Pat. No. 5,170,018 issued to Potier discloses the use of absorptive material, such as cork or Sorbothane™ from Sorbothane, Inc. (Kent, Ohio), deployed in a non-metallic housing above and below a seismic receiver in cross-well or RVSP surveys.
Most of these prior art methods of isolating seismic wave propagation in borehole fluids are for use in cross-well, VSP or RVSP applications, in which the source and the receivers are not in the same borehole. If the receivers and the source are in the same borehole, the tube waves will be substantially stronger.
Being able to perform seismic profiling with the source and receivers in the same borehole offers many advantages. Therefore, it is desirable to have seismic sources that can be used with the receivers in the same borehole and to have efficient methods for minimizing the propagation of seismic waves in the borehole fluids.